1. Field of the Invention
The present invention relates to a dielectric ceramic which is used in electronic components that are used in high frequency region such as microwaves and milliwaves, a method for producing the dielectric ceramic, and an electronic component using the dielectric ceramic.
2. Description of the Related Art
In recent years, a high-frequency band referred to as so-called quasi-microwaves of about several hundred MHz to several GHz is used for mobile communication equipment such as mobile phones, the demand for which is ever increasing. Therefore, electronic components that are used in mobile communication equipment, such as capacitors, filters, resonators and circuit boards, are also required to have general characteristics appropriate for the use in high-frequency bands.
Circuit boards, which constitute one class of the electronic components used in high-frequency bands, include conductors such as electrodes or wiring (hereinafter, referred to as “conductor materials”), and have built-in LC filters which are each formed from a combination of a magnetic body and a dielectric body, built-in capacitors which are each formed from a combination of a high dielectric constant material and a low dielectric constant material, and the like. Thus, these circuit boards construct circuits with LC filters, capacitors and the like.
In a circuit board, it is needed to lower the relative permittivity ∈r of the board in order to reduce signal delay that is attributable to the wiring capacitance in the wiring layer of the circuit board. Furthermore, since high frequency signals are not to be attenuated in a circuit board, it is necessary to make the Qf factor of the substrate large (that is, reducing the dielectric loss). Therefore, a dielectric material having low relative permittivity ∈r in the use frequency and having a large Qf factor is demanded as a material for circuit boards. Q represents the reciprocal of tangent of the loss angle δ, tan δ, which is the difference between the actual phase difference between current and voltage and the ideal phase difference of 90° between current and voltage in a dielectric substance, and f represents the resonant frequency. The Qf factor is represented by the product of the quality factor, Q=1/tan δ, and the resonant frequency f, and when the Qf factor increases, the dielectric loss decreases.
Generally, materials having low dielectric constants in many cases exhibit small dielectric losses, and those materials are used for devices in the microwave region. For example, an LC filter is formed by simultaneously firing a high dielectric constant material and a low dielectric constant material. In an LC filter, when a low dielectric constant material having a high Q factor is used so that the ceramic material at the part constituting the L unit can have a high self-resonant frequency, and a high dielectric constant material having favorable temperature characteristics is used for the C unit, an LC element having a high Q factor and having favorable temperature characteristics can be realized.
As such dielectric materials, for example, a dielectric ceramic composition containing forsterite (Mg2SiO4) as a main component, and zinc oxides, boron oxides, alkaline earth metal oxides, copper compounds and lithium compounds as additives (hereinafter, referred to as a “forsterite-based composition”), has been suggested (see, for example, Japanese Laid-open Patent Publication No. 2009-132579). The forsterite composition can be fired at a temperature lower than the melting point of metal silver (Ag) or an Ag-based alloy (hereinafter, referred to as an Ag-based metal), which are conductor materials, and thus the mechanical strength can be increased when the composition is fabricated into a substrate. Therefore, the forsterite composition is appropriate as a dielectric material for circuit boards.
In other wards, since the sintering temperature of a forsterite-based composition is about 1,000° C. or lower, and this sintering temperature is lower than the sintering temperatures of conventional dielectric ceramic compositions, the forsterite-based composition has a lower melting point as compared with the conductor materials that have been traditionally used, such as palladium (Pd) or platinum (Pt), and has a low electrical resistance, and an inexpensive Ag-based metal can be used as a conductor material. Accordingly, a forsterite-based composition can be co-fired at a low temperature (a temperature lower than the melting points of Ag-based metals). Therefore, when a forsterite-based composition is used to form a circuit board, the forsterite-based composition is used as a low temperature co-fired ceramic (LTCC) which can be co-fired with a conductor material such as an Ag-based metal, and a circuit with LC filters, capacitors or the like can be fainted in different layers.
However, conventional forsterite-based compositions have a problem that because sintering aids such as zinc oxide, which are added to forsterite-based compositions as sintering aids, remain unreacted after the main firing process, the resulting dielectric ceramics have large dielectric losses.
Furthermore, there is also a problem that when the amount of addition of sintering aids is reduced, and thereby the amount of unreacted sintering aids is decreased, sinterability of the fired material that is obtained by firing a forsterite-based composition is impaired, the dielectric loss of the substrate is increased, and thereby the mechanical strength of the substrate is deteriorated.
Accordingly, there is a demand for a dielectric ceramic having excellent dielectric properties, which may be obtained by preventing any unreacted sintering aids from remaining behind after firing a forsterite-based composition at a low temperature, and also by securing sinterability by causing the sintering aids to undergo complete reaction.